How can I modify the frame rate without changing overall animation speed?
You can change the timing of an animated scene without changing the frame rate. In the rendering settings Dimensions there is a time remapping section:



It works like this:

The values are in frames

For example:

If you want the whole animation to run twice as fast you set the old to 100 and new to 50.

Animated values that took 100 frames now will happen in 50

To make the animation last twice as long (slower) map old to 100 and new to 200.